It has been authoritatively estimated that the conventional american fireplace extracts as much warm air from the room which it is intended to heat as it introduces into the room. This phenomenon occurs since the principal source of air for combustion is the room air itself. All of which tends to be at the room temperature. The constant drawing of room air causes that air to be replenished by outside air seeping in through cracks or drawn from other rooms, and is in actuality counter-productive. Drawing of the draft from the room does aid in minimizing smoke entrance into the room but this can be achieved successfully by the use of proper chimney design, flue damper, and care in operation.
In recent years of increasing costs of fuel oil there has been a renewal of interest in the room fireplace as a source of heat. This increase in interest has been further augmented by recent advances in the design of fireplace grates which draw air from the room, heat it, and return it, either using convection currents, or in certain cases using air pumps. Again, such system fails to take into account that combustion air is drawn from the room and in quantities far in excess of that which is heated in the grate and returned.
It has been recognized by others in the past that it is desirable to draw combustion air from the exterior and a number of special fireplace designs have been produced with that thought in mind. Each of them have added complications.
None of the foregoing solve the problem for the existing fireplace without any provision for drawing outside air. Many of these existing fireplaces however do include an ash trap which characteristically includes one or more hinged doors which may be depressed to allow ashes to be swept or dragged to a chamber below or to the rear of the fireplace. The ash trap door is used seldom, if ever, and has no operating function when the fireplace is in operation.